How to manipulate cart in an app using external inventory?

[GregL]

Hi Everyone,

i am trying to develop an app which will use a kind of external inventory.
I created an app-theme-extension and created a block to check if a unit of the product exists in my inventory and if yes, i will add it to the cart and proceed to the purchase. See the code of my block below. 

My code is not working and i got the error : Uncaught (in promise) ReferenceError: cart is not defined
isn't it possible to use cart as an object ? 

Do you think this is the best approach ? 

<script>

// This script will check if the product is in stock in the external inventory system.

// If it is, it will add the product to the Shopify basket.

async function checkInventory() {

// Get the product ID from the Shopify product.

console.log("Product id:",{{product.id}});

const productId = {{product.id}};

// Make a request to the external inventory system to check the inventory for the product.

const response = await fetch('https://example.com/api/inventory/check/' + productId);

// Check the response status code.

if (response.status === 200) {

// The product is in stock. Add the product to the Shopify basket.

cart.items.push({

product_id: {{ product.id }},

quantity: 1

});

} else {

// The product is not in stock. Show an alert message to the customer.

alert('The product is not in stock.');

}

}

</script>

<button

typ="button"

class="shopify-payment-button__button"

style="background-color:{{ block.settings.background-color }};color:{{ block.settings.color }}"

onclick="checkInventory()"

>

{{ 'myapp.addToBasketButton.label' | t }}

</button>

{% schema %}

{

"name": "My app Product",

"target": "section",

"settings": [

{ "type": "product", "id": "product", "label": "product", "autofill": true },

{ "type": "color", "id": "color", "label": "Button Color", "default": "#FFFFFF" },

{ "type": "color", "id": "background-color", "label": "Button background Color", "default": "#168F30" }

]

}

{% endschema %}